1. Field of the Invention
This invention relates to a droplets forming device which discharges recording liquid, generally called "ink". More particularly, it is concerned with a droplet forming device applicable to the so-called "ink-jet recording system" which performs recording of an image with ink droplets.
2. Description of the Prior Art
Of various kinds of recording systems which are known, the so-called "ink-jet recording system" is recognized to be an extremely useful recording system. This ink-jet recording system is of a non-impact type which generates substantially no noise at the time of the recording, is able to perform recording a high speed recording, and yet is able to perform such recording on plain paper without requiring any particular image fixing treatment.
Various systems have heretofore been proposed for this ink-jet recording method, some of which have already been commercialized after repeated improvements, and some others are still under development for practical uses.
The ink-jet recording method performs recording of an image on an image recording member such as paper, etc. by sputtering or ejecting droplets of the recording liquid (hereinafter simply called "ink") by various working principles. This ink-jet recording method may be classified into the following two type.
The first type is the so-called "continuous system", in which small droplets of ink are continuously ejected from a nozzle (or nozzles), and only those ink droplets which are required for recording are selected out of the discharged droplets and guided to the surface of the recording member where they are adhered to complete the image recording. The other type is so-called "ink-on-demand system", in which the ink droplets are ejected or sputtered from the nozzles toward the surface of the recording member only when recording becomes necessary.
For putting this latter method into practice, there has already been proposed an ink-jet head in a laid-open Japanese patent application No. 54-51837. The ink-jet head device as proposed in this laid-open patent application comprises an ink; chamber having nozzles for ejecting liquid ink feeding reservoir; a heat generating member to heat the liquid ink in the ink chamber to develop foam within the ink chamber and to cause a pressure increase in the ink; and a cooling device to cool the abovementioned heat generating member. This disclosed invention has, as its principal technical aim, the prevention of dropping of ink unexpectedly from the nozzles as well as clogging of the nozzles.
Such ink-jet recording system, however, still has various disadvantages in view of the fact that the device should inevitably use the cooling device; represented by a peltier effect element, in addition to the heat generating member to both be driven for ejecting the ink droplets. In other words, the following inconveniences can be pointed out in the abovementioned system which essentially requires a cooling device.
Firstly, as it is necessary to cover substantially the entire region of the ink-jet head with the cooling device, the construction of the actual device inevitably becomes complicated, and much more labor and skill are required for its manufacture.
Secondly, since electric power should be used for operating the Peltier effect element (Cooling device), besides the operation of the heat generating member, the recording system is disadvantageous in respect of its energy efficiency.
Thirdly, considerably high technology is required for efficiently controlling the heat applying and heat absorbing actions by operation of the heat generating member and the cooling device, respectively, practice of which also accompanies considerable difficulty.
In the fourth place, since the ink in the ink chamber is rapidly cooled or over-cooled an many occasions by the cooling device, there tends to readily occur readily excessive backwardness of the meniscus at the tip end of the nozzles, which often invites failure in the ejection of the ink droplets.
In the fifth place, since the recording system repeats the heating operation and subsequent rapid cooling operation, the recording device is liable to be easily broken, hence the durability in a commercial device is not satisfactory.